The almost indestructible disks would contain a record of human existence -- perhaps for alien life forms

Scientists in Germany and The Netherlands have determined that by using tungsten and silicon nitride as a storage medium, they can store data that will last from a million to a billion years. They propose using the medium to retain data on human physiology that could be seen long after the human race is extinct.

The nearly indestructible disks were created by the MESA Institute for Nanotechnology at the University of Twente in The Netherlands and The Freiburg Institute for Advanced Studies in Freiburg, Germany.

The disk consists of a 338-nanometer (nm) thick layer of silicon nitride on top of a silicon wafer. A 50 nm layer of tungsten is patterned into QR (Quick Response) codes using optical etching lithography on top of the silicon nitride wafer.

The QR codes, which had line-widths of about 100nm thick, were etched using an argon ion laser beam. A 224nm layer of nitride was then deposited on top of tungsten patterns to help protect them.

To give some idea of how thin the materials used for the storage medium are, an 8.5-by-11-in. sheet of paper is about 100,000 nanometers thick. So the silicon wafers could only be read with the use of a microscope.

The researchers chose QR codes, which are commonly used in consumer advertising today, because they can be easily read by machines using simple scanners. For example, QR codes can be read by today's camera-enabled smartphones.

The materials used are highly resistant to heat. Silicon nitride has a melting point of 3,452 degrees Fahrenheit, and tungsten has a melting point of 6,170 degrees Fahrenheit.

What the researchers created was a write-once, read many (WORM) disk platter that was nearly indestructible. WORM technology was used because the project's intent was to create an indelible record that would stand the test of time over millions of years, even after the human race may have become extinct.

The storage medium was made for the Human Document Project, an initiative started in 2002 by a consortium of European institutions. The project's purpose was to create a digital library that would stand the test of time after human beings either migrated away from Earth or became extinct.

To prove the disk could weather the test of time, the scientists etched the microscopic QR codes into the tungsten surface of the disk and then heat tested them to simulate age and what might happen to the material, for example, it were in a building fire.

Initial calculations showed it is possible to store data for more than a million years and up to a billion years, the researchers stated in a published paper.

The researchers first age-tested the disk by heating it to 445 Kelvin (341 Fahrenheit) for one hour.

"We observe no visible degradation of the sample, which indicates that this sample would still be error free after 1 million years," the researchers stated.

The disk was then tested at up to 763 Kelvin (913F) for two hours, which caused some degradation.

Optical microscope images of the same QR code left: after fabrication, center: after two hours at 613 K and right: 2 hours at 763 K (Image: MESA Institute and University of Twente)

As the temperature was increased, the top layer of silicon cracked and reduced the number of readable QR codes. While not readable by the QR code algorithm, the QR codes themselves were not "visibly" damaged and the tungsten was still present in the material. Overall, the QR codes lost about 7% of their readable data at higher temperature tests, the researchers said.

"The misreading of the information is caused by the readout using an optical microscope without a monochromatic light source. The images are taken using a top mounted camera and contain a multitude of colors, caused by the variation in [the silicon's] thickness due to the cracking," the researches wrote. "The very simple detection software was unable to correctly assign a black or white color to multitude of colors caused by the cracking of the top silicon-nitride layer."